About 11m households on default tariffs have seen their bills capped since 1 January, narrowing the gap between the best and worst deals to £150-£200, compared with more than £250 previously, according to switching site MoneySuperMarket.

The smaller savings had led the energy regulator to admit that switches may reduce by as much as 40%, which would wipe out the growth in recent years.

Lawrence Slade, chief executive of Energy UK, said: “My hope remains that, with the recent introduction of the price cap, we don’t see this element of competition undermined and switching levels fall, as is predicted in Ofgem’s impact assessment.”

Across the year, a net 1.7 million customers moved to small and mid-sized suppliers, rather than the big six that hold four-fifths of the market.

Small suppliers dominate the top of a new league table of customer satisfaction, with Which? reporting Octopus Energy the best in the market.

What is the energy price cap and how does it work?

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Bucking the trend was another small firm, Solarplicity – it came bottom of the consumer group’s ranking which surveyed 8,000 people on suppliers’ value for money, customer service and billing accuracy.

The big six occupied the bottom third of the table, with Scottish Power the worst of the six and SSE the best.

Rachel Reeves, Labour MP and chair of the cross-party business, energy and industrial strategy committee, said: “The Which? survey highlights once again that the big six are miserably failing their customers. Having ripped off loyal customers on SVTs (standard variable tariffs) for far too long, this survey shows that they aren’t delivering a service which is up to scratch either.

“Customers should continue to shop around because they cannot rely on energy suppliers giving consumers a good deal or delivering the quality customer service which they deserve.”